The present invention relates to plasma generators, and more particularly, to a method and apparatus for generating a plasma to sputter deposit a layer of material in the fabrication of semiconductor devices.
Plasmas have become convenient sources of energetic ions and activated atoms which can be employed in a variety of semiconductor device fabrication processes including surface treatments, depositions, and etching processes. For example, to deposit materials onto a semiconductor wafer using a sputter deposition process, a plasma is produced in the vicinity of a sputter target material which is negatively biased. Ions created adjacent the target impact the surface of the target to dislodge, i.e., "sputter" material from the target. The sputtered materials are then transported and deposited on the surface of the semiconductor wafer.
Sputtered material has a tendency to travel in straight line paths, from the target to the substrate being deposited, at angles which are oblique to the surface of the substrate. As a consequence, materials deposited in etched openings including trenches and holes of semiconductor devices having openings with a high depth to width aspect ratio, may not adequately coat the walls of the openings, particularly the bottom walls. If a large amount of material is being deposited, the deposited material can bridge over causing undesirable cavities in the deposition layer. To prevent such cavities, sputtered material can be redirected into substantially vertical paths between the target and the substrate by negatively biasing (or self biasing) the substrate and positioning appropriate vertically oriented electric fields adjacent the substrate if the sputtered material is sufficiently ionized by the plasma. However, material sputtered by a low density plasma often has an ionization degree of less than 10% which is usually insufficient to avoid the formation of an excessive number of cavities. Accordingly, it has been found to be desirable to increase the density of the plasma in order to increase the ionization rate of the sputtered material and to decrease the formation of unwanted cavities in the deposition layer. As used herein, the term "dense plasma" is intended to refer to one that has a high electron and ion density, in the range of 10.sup.11 -10.sup.13 /cm.sup.3.
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil surrounding the plasma induces electromagnetic currents in the plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance matching network, such that the coil acts as the primary winding of a transformer. The plasma acts as a single turn secondary winding of the transformer.
One approach has been to dispose the coil on the interior surface of the shield wall of the processing enclosure, or chamber, so that the plasma generating coil surrounds the volume, or space, enclosed by a surface that extends between, and contains, the edges of the target and the workpiece support surface. Thus, when the coil is installed within the chamber in accordance with this approach, it is positioned immediately adjacent to the chamber shield wall. Configuring the coil in the manner described above is based on the assumption that this promotes plasma uniformity adjacent the workpiece and that the uniformity, across the workpiece area, of the treatment effected with the sputtered ionized material depends on the plasma uniformity.
However, experience has shown that even when the plasma density is extremely uniform, the amount of deposition material delivered per unit workpiece surface area and per unit time, and hence the resulting thickness of the deposited film, tends to be greater at the center of the workpiece surface area than at the edge thereof, and diminishes progressively from the center to the edge.
Thus, although ionizing a relatively large portion of the deposition material facilitates deposition of material into high aspect ratio channels and vias, many sputtered contact metals have a tendency to deposit more thickly in the center of the wafer as compared to the edges. This "center thick" deposition profile is undesirable in many applications where a uniformity of deposition thickness is needed. It is believed that in the case of high density plasmas, the high gas pressures associated therewith produce a scattering effect which contributes to this "center thick" tendency.
To compensate for this center thick tendency, material may be sputtered from the coil itself. When the coil is located close to the chamber shield wall as discussed above, the coil will be closer to the periphery of the workpiece and farther away from the center of the workpiece. As a consequence, the deposition profile for material sputtered from the coil tends to be somewhat "edge thick." As a result, the center thick tendency of the target sputtered material can be compensated.
However, it is recognized that sputtering material from the coil may not be optimal for all applications. Accordingly, another approach for improving sputter deposition uniformity of ionized deposition material is needed.